Port Injection System For Reduction Of Particulates From Turbocharged Direct Injection Gasoline Engines

ABSTRACT

The present invention describes a fuel-management system for minimizing particulate emissions in turbocharged direct injection gasoline engines. The system optimizes the use of port fuel injection (PFI) in combination with direct injection (DI), particularly in cold start and other transient conditions. In the present invention, the use of these control systems together with other control systems for increasing the effectiveness of port fuel injector use and for reducing particulate emissions from turbocharged direct injection engines is described. Particular attention is given to reducing particulate emissions that occur during cold start and transient conditions since a substantial fraction of the particulate emissions during a drive cycle occur at these times. Further optimization of the fuel management system for these conditions is important for reducing drive cycle emissions.

This application is a Continuation of U.S. patent application Ser. No.15/959,341 filed Apr. 23, 2018, which is a Continuation of U.S. patentapplication Ser. No. 15/214,533 filed Jul. 20, 2016 (now U.S. Pat. No.9,976,496 issued May 22, 2018), which is a Continuation of U.S. patentapplication Ser. No. 14/391,906 filed Oct. 10, 2014 (now U.S. Pat. No.9,435,288 issued Sep. 6, 2016), which is a 371 of InternationalApplication No. PCT/US2013/073334 filed Dec. 5, 2013, which claimspriority of U.S. Provisional Patent Application Ser. No. 61/734,438,filed Dec. 7, 2012, the disclosures of which are incorporated herein byreference in their entireties.

BACKGROUND

Particulate matter (PM) emissions from turbocharged direct injectionspark ignition engines using gasoline and gasoline-ethanol blends are anincreasing concern. A key factor that produces this problem is poormixing from directly injected fuel (fuel that is directly introducedinto the engine cylinder as liquid). The particulate emissions problemis increased in turbocharged engines due to increase in the absolutepressure in the cylinder. The concern about particulate emissionsrelates to both the total mass of the particulates and the number ofparticulates. Meeting anticipated European requirements for reducing thenumber of particulates appears to be especially demanding.

Port and direct injection have complimentary advantages. Due to bettermixing, reduced wall wetting and improved evaporation of the fuel, bothparticulate mass/km and number of particulates/km emissions from portinjection, where the fuel is introduced in a region outside thecylinders, are typically less than one tenth those from direct injection(when the fuel is introduced as a liquid into the cylinder). On theother hand, direct injection provides better knock resistance due togreater evaporative cooling of in cylinder charge and thus allowsoperation at higher levels of torque for a given engine displacementand/or compression ratio. Direct injection can also be used to providebetter control of fueling and to further increase efficiency by use ofstratified operation to enable lean operation at low loads.

SUMMARY

The present invention describes a fuel-management system for minimizingparticulate emissions. The system optimizes the use of port fuelinjection (PFI) in combination with direct injection (DI), particularlyin cold start and other transient conditions.

An important aspect of using the combination of port and directinjection to reduce particulate emissions is the employment of controlsystems, such as those described in U.S. Pat. No. 8,146,568, to minimizethe fraction of the fuel that is directly injected into the engine whilealso preventing knock as the torque is increased. Both closed loopcontrol with a knock detector and open loop control with a look up tablecan be employed. These control systems can provide better mixingthroughout a drive cycle with essentially no compromise in engineefficiency and performance. The engine can be operated with port fuelinjection alone, direct injection alone or a combination of port anddirect injection. The fraction of fuel that is port fuel injected can becontrolled to prevent knock as a function of both torque and enginespeed since the engine speed also affects the onset of knock.

In the present invention, the use of these control systems together withother control systems for increasing the effectiveness of port fuelinjector use and for reducing particulate emissions from turbochargeddirect injection engines is described. Particular attention is given toreducing particulate emissions that occur during cold start andtransient conditions since a substantial fraction of the particulateemissions during a drive cycle occur at these times. Furtheroptimization of the fuel management system for these conditions isimportant for reducing drive cycle emissions.

BRIEF DESCRIPTION OF FIGURES

FIG. 1 illustrates an engine with two injectors; a port fuel injectorthat provides fuel to a region in the manifold, which is outside of thecylinder, one per cylinder, and a direct injector that introduces fueldirectly into the cylinder, one per cylinder, with the ratio of fuelinjected by each injector changed in an optimized way so as to reduceparticulate emissions;

FIG. 2 is a schematic of an engine of FIG. 1, and shows an enginecontrol system where information about the engine condition, such astemperature, that influences particulate formation and the load demandfrom the operator are used to determine the ratio of fuel injectedthrough the port fuel and the direct injectors;

FIG. 3 illustrates an engine system with both port fuel and direct fuelinjectors, as shown in FIG. 2, with additional engine performancesensors that measure the engine operation and/or emissions. This type ofengine performance sensor could be a knock sensor, a misfire sensor, anexhaust sensor, or any combination of the three, where the exhaustsensor could monitor, in real time or near real time, the particulateemissions;

FIG. 4 shows the engine system of FIG. 3 with the introduction of sparktiming so that spark retard may be adjusted to allow more PFI use whenneeded to reduce particulate emissions;

FIG. 5 illustrates an engine system with both port fuel injectors anddirect injectors, as shown in FIG. 2, with an additional injector or setof injectors upstream from the compressor blades of the booster, whichcould be a turbocharger or a supercharger; and

FIG. 6 shows the engine system of FIG. 3 with the introduction of valvetiming (inlet, exhaust or both), which may be adjusted to allow more PFIuse when needed to reduce particulate emissions.

DETAILED DESCRIPTION

As described above, the present invention uses an engine control systemto monitor and regulate the operation of the engine to both maximizeengine performance and efficiency, and to minimize particulate emission.The engine control system may affect the fraction of fuel that isintroduced using PFI and DI, based on engine conditions, particulateemissions and other factors.

Closed loop control, open loop control or a combination of open andclosed loop may be used to optimize particulate emissions reductionwhile providing the desired amount of knock control. Various embodimentsof these control systems are shown in the accompanying figures.

FIG. 1 shows a diagram of an engine system that has two sets ofinjectors, a port fuel injector 10 that introduces fuel into a regionoutside of the cylinder, such as manifold 15, one per cylinder, and adirect injector 20 that introduces fuel directly into the cylinder 30 asa liquid, one per cylinder. The engine may be a spark ignited engine,although other types of engine operation, such as HCCI (HomogeneousCharge Compression Ignition) or its variants (such as RCCI, ReactionControlled Compression Ignition, or PCI, Premixed Compression Ignition,other low temperature combustion processes) may be employed. The enginecontrol unit 40 uses information that relates to particulate emissionsand adjusts the ratio of the amount of fuel that is port injected to theamount of fuel that is directly injected so as to reduce particulateemissions. In this embodiment, the engine control unit 40 does not haveany feedback about engine performance. Thus, the information related toengine performance could be in the form of a look up table or a formula.The look up table or tables may use the engine map to estimate engineperformance.

FIG. 2 shows a diagram of the engine control system of FIG. 1 duringengine operation. The engine control unit 40, also referred to as thefuel management system, determines, in response to the load demand fromthe operator and using information about the engine condition 50 thatinfluences particulate emissions, the proper operation of the injectors10, 20. The engine condition 50 may include information about enginetemperature, coolant temperature, time since start, status of the fuelpool in the manifold, and other factors that influence particulateemission.

Using these inputs, the engine control unit 40 may regulate the ratio ofthe fuel that is injected through the port fuel injector 10 as comparedto that directly injected into the cylinder 30, and other injectioncharacteristics. For example, the engine control unit 40 may also adjustthe start of injection, the end of injection, the injection rate. In thecase of the direct injector 20, injection rate may be adjusted throughmanagement of the fuel rail pressure, for example, or through control ofthe pulse width for a system that uses pulse-width modulation. Theengine control unit 40 may also control the valve timing, allowing forthe injection of port fuel injection through an open inlet valve, wherethe PFI fuel is introduced into the cylinder as liquid droplets, insteadof as a gas vaporized by the hot valves. All of these various actionsmay be referred to as “injector operation”.

FIG. 3 shows the engine system of FIG. 2, with the addition of engineperformance sensors 60 that measure the engine operation and/oremissions. An engine performance sensor 60 may be, for example, a knocksensor, a misfire sensor, an exhaust sensor, or any combination of thethree. Information from the knock sensor can be used to reduce therelative amount of fuel that is port fuel injected as the torque isincreased so as to prevent knock. In some embodiments, the exhaustsensor could monitor, in real time or near real time, the particulateemissions. The exhaust sensor may determine the number of particulatesor the mass number of the particulates. The exhaust sensors could beelectrostatic (measuring charge), optical (counting particles orextinction of an optical signal) or mass based (where the presence ofthe particulate mass changes a measurement, such as the resonantfrequency of a system). This engine system may regulate the injectionoperation based on information from these engine performance sensors 60.Thus, while the embodiment of FIG. 2 uses a lookup table or equation toestimate engine performance, this embodiment may use information fromthe engine performance sensors 60.

FIG. 4 shows a diagram of a control system, similar to FIG. 3, whichcontrols the ratio of the amount of fuel that is port fuel injected tothe amount of fuel that is directly injected. The engine control unit 40may receive information on engine condition and engine performance. Theengine control unit 40 also controls the amount of spark retard 70 thatis used to enable a higher relative amount of fuel that is portinjected. Using this information, at some times, such as in the coldstart or warm restart periods, the ratio of the amounts of port injectedfuel to directly injected fuel can be greater than would be used atother times in the drive cycle at the same value of engine torque andspeed. Spark retard may be increased to prevent knock that wouldotherwise occur. The maximum ratio of port fuel injection to directinjection would be determined by the maximum amount of spark retard thatcould be used without an unacceptable reduction in engine efficiency andperformance. Spark retard, with the resulting decrease in efficiency,can be used at these times to substantially reduce particulate emissionswith only a minimal effect on engine efficiency during a drive cycle.Large spark retard during cold start or warm restart can have theadditional advantage of rapid engine and catalyst heating (as the engineis less efficient). The amount of spark retard can be limited so thatthe engine efficiency and performance do not fall below selected values.The engine control unit 40 of FIG. 4 also controls injector operation,as described above.

FIG. 5 shows the engine system shown in FIG. 2, with both port fuelinjectors 10 and direct injectors 20. In this embodiment, an additionalinjector or set of injectors (not shown), referred to as upstreaminjectors, are disposed upstream from the compressor blades of thebooster 80, which could be a turbocharger or a supercharger, are used.It may be advantageous to introduce the port fuel injected fuel upstreamfrom the turbocharger 80. The high velocity of the blades induces strongturbulence in the air/fuel mixture and blasts the impinging liquid fuel,improving atomization of the fuel. One advantage of injection upstreamfrom the turbocharger 80 is reduced number of injectors, as a singleinjector is needed. In order to improve air/fuel ratio, it may bepossible to adjust the fuel that is introduced into every cylinder 30.It may be possible to combine a single injector upstream from theturbocharger or turbochargers 80 with additional port fuel injectors 10that provide fuel to the inlet valves of the cylinders. Even underconditions when the blades of the compressor are not spinning very fast,there are enough surfaces for allowing evaporation of the fuel duringtransients that would allow for sufficient fuel vapors to enter thecylinder 30. The rest of the fuel will be deposited on the walls of themanifold 15 and will be evaporated later in the engine operation. Thus,the compressor in the booster 80 and the fuel injector operate as acarburetor, allowing for increased flexibility of control of the engineoperation for controlling particulate emissions. The advantages of thisconfiguration are rapid and effective evaporation of the fuel, allowingthe fuel injected through the manifold to be in the vapor phase. Inaddition to smaller particular matter formation in cylinder, hydrocarbonemissions, especially during cold start and during transients, may alsobe decreased. Thus, in this embodiment, the engine control unit 40 maycontrol injector operation of the direct injectors 20, the port fuelinjectors 10, and the additional upstream injectors.

FIG. 6 shows the engine system of FIG. 3 with port fuel injectors 10 anddirect injectors 20 where variable valve timing 90 is used to allow ahigher fraction of fuel provided by port fuel injection duringconditions with increased particulate generation would occur. In thisembodiment, the engine control unit 40 may employ variable valve timingto compensate for the reduced direct injected fuel. For example, atcertain times, the use of direct injected fuel may cause excessiveparticulate emission. In these times, rather than using the requisiteamount of direct injected fuel, variable valve timing 90 is used toinsure no-knock operation. For instance, it may be possible to operatethe engine with increased value of residuals for knock control, throughappropriate choice of valve timing.

These figures show the sensors 50, 60 can be employed to monitor engineconditions and/or engine performance. One or more of these types ofsensors can be used in an embodiment. Additionally, these figures showvarious compensation techniques that are used to allow the use of anamount of port injected fuel that would otherwise cause knock. Thesecompensation techniques include, but are not limited to valve timing,injector timing, upstream injectors, and spark timing. Although eachfigure shows the use of only one of these compensation techniques, it isnoted that more than one compensation technique may be used in a singleembodiment. For example, spark timing and valve timing may both beemployed in some configurations. Therefore, the figures should not beconstrued as limiting the disclosure to those particular configurations.Rather, these figures simply show various components that may be used.

Compensation Techniques

The above description discloses various compensation techniques, such asspark timing, and variable valve timing. However, other compensationtechniques may also be used.

In addition to the use of variable valve timing, the use of on-demandcylinder deactivation can also be used in combination with twoinjectors. During transients at relatively low load, some of thecylinders can be deactivated. If the exhaust valves are not opened oronly open partially or during a short time, during the following cycle,the high pressure, hot gases in the cylinder can be used to helpvaporize the port fuel injected fuel in that same cylinder. When theinlet valve opens following the cycle with exhaust restricted, highpressure, heated air enters the manifold and helps vaporize the gasolinein the manifold and on the valves. The process can be repeated in thesame cylinder, and the process can be arranged so that it occurs also,in a staggered manner, at other cylinders.

In the same manner, it would be possible to have a cycle with restrictedinlet valve opening, with limited ingestion of port fuel, that goesthrough compression, power stroke and exhaust cycle without firing thespark and without opening inlet or exhaust valves. In the next intakevalve opening, the air leaving the cylinder is hot and can vaporize thefuel in the valve or the manifold. If done in all cylinders, the engineactually fires only on about half the cycles. Alternatively, in thefollowing cycle, direct injection can be used to inject into a hotcylinder charge, facilitating droplet evaporation and minimizingpotential wall wetting and production of particulate matter.

Another compensation technique that may be used is as follows. It may bepossible to adjust the pressure of the direct injected fuel, such as byadjusting the pressure in the common fuel rail. This serves to minimizethe impingement on the combustion chamber walls (cylinder walls, pistonand valves), especially at transient conditions or at cold start. If thepressure is difficult to manage, a strategy that can be used to avoidthe injection of very high speed droplets that will wet the wall is touse PWM (Pulse Width Modulation) of the direct injector. Injectors havetransients during turning on and turning off that can be used to modifythe conditions of injection of the fuel, even at constant rail fuelpressure. Through short pulses allowed by PWM fuel control, it would bepossible to inject short sprays of fuel that modify the jets interactionwith the gas in the cylinder, for example, by preventing them fromreaching the far wall. Although atomization may not be as good, theproduction of particulate matter could be reduced by the use of multiplepulses, and in particular, short pulses that are dominated by transientbehavior of the injector. Because of the short duration of the timeswhere PM is produced, the injectors can be prevented from overheatingdue to the large number of pulses required that heat the solenoid valve,the piezoelectric components or other components from the direct fuelinjector.

Another compensation technique includes open-valve port fuel injection.The fueling system may use port fuel injection, when the intake valve isopen, as an additional means for reducing particulates. Port fuelinjection with the intake valve open enables some of the fuel to enterthe cylinder in the liquid form and provide some vaporization cooling.This would provide knock suppression from charge cooling that couldreplace that which is typically provided by direct injection. Althoughless knock suppression by charge cooling is obtained for a given amountof fuel, the net result could be less particulate formation, especiallyduring transients. The same port fuel injector could be used for bothconventional port fueling injection with the intake valve closed and foropen-valve port fuel injection where increased knock resistance could beprovided.

Engine Control Unit Operation

In addition to information that is used to determine how much port fuelinjection can be used without encountering knock, the fuel managementsystem may also employ additional information and requirements tocontrol the relative amounts of fuel that are port and directlyinjected. This may include information related to cold start, such asengine temperature, and information that determines when stratifiedinjection would be used. Additional information for controlling of theratio of directly injected fuel to port injected fuel could be providedby sensors that determine particulate mass and number, as describedabove.

The control system uses some or all of this information in conjunctionwith the requirement to avoid knock to determine the ratio of port fuelinjection to direct injection that is used at a given value of torqueand speed. The ratios may be different depending on whether the primaryobjective is reduction of engine particulate mass or engine particulatenumber. It would also be possible to trade-off particulate matter andhydrocarbon emissions. In addition to the combustion information,information on the nature of the fuel can be used to adjust the ratio ofthe direct injected and port injected fuels. If the fuel has substantialcontent of saturated hydrocarbons or oxygenates (alcohols), the ratio ofdirect injected to port fuel injected fuel can be adjusted to providebest emissions. The nature of the fuel can be determined by the pasthistory of the vehicle. For example, when the fuel contains asubstantial fraction of alcohols, direct injection of the fuel can beused to decrease simultaneously hydrocarbon emissions and particulateemissions, compared to the case with port fuel injection of the samefuel, which produces little particulate matter but substantialhydrocarbon vapor during cold start and warm restart.

In addition, as described above, closed loop control using measurementsof particulate mass and number can be employed. The fuel managementsystem can control the fraction of fuel that is introduced by port fuelinjection so to minimize particulate mass or number or some combinationof the two.

The parameters that are inputs to the control system can include enginetemperature; time after ignition; rate of change of engine fueling; rateof change of engine speed; rate of change of torque, engine speed andengine torque; fuel composition. The control system could also take intoaccount the use or non-use of stratified direct injection.

These various embodiments, such as shown in FIGS. 1-6, can be used todecrease particulate emission during various critical times. Some timeswhere particulate emission is known to be excessive are engine coldstart and engine shutdown. Thus, the engine control unit describedherein can be used to adjust the engine operating conditions, such thatwhen particulate emissions would be high, the control system will adjustthe port fuel injection to directly injected fuel ratio so as to use agreater fraction of port injected fuel than would be used if particulateemissions were not a concern.

Cold Start

For example, when the engine is operated during cold start at a value oftorque where a substantial amount of directly injected fuel wouldotherwise be needed to prevent knock, one or more compensationtechniques may be introduced to reduce the relative amount of directlyinjected fuel without resulting in knock.

One such compensation technique is spark retard. Increased spark retardresults in decreased efficiency. However, operation with reduced fuelefficiency during cold start and other transients, for most drivingcycles, does not substantially affect the overall fuel efficiency.Moreover, the lower efficiency of engine operation during the short coldstart transient can have the beneficial effect of increasing the rate ofwarm-up of engine and aftertreatment components.

During cold start, particulate control can be achieved through similarmeans used to minimize the production of hydrocarbons that would resultin fast engine and catalyst warm-up. Thus, spark retard beyond what isrequired to control knock, valve timing adjustment to maximize pressurein the cylinder and residuals, and possible injection of a smallfraction of the fuel through the direct injector can be used to minimizethe formation of particulates during the cold start. Either the timingof the inlet valve, the exhaust valve or both can be adjusted by theoperating system in order to minimize the production of particulates.The effect can be adjustment modification of the pressure, compositionor flow dynamics in the cylinders. In the case of the flow dynamics, theflow can be modified from that during steady state conditions (eithertumble or swirl or quiescent) so that the resulting flow during thetransient minimizes wall wetting and production of particulates. In thecase of multiple inlet valves or multiple exhaust valves, the timing ofeach valve may be adjusted independently in order to modify the flow incylinder.

Another control feature for reducing particulate emissions during coldstart is to reduce the amount of direct injection that is used in orderto minimize hydrocarbon vapor emissions. The fueling of the engine atthe earliest time in the cold start period could be by direct injectionalone or mainly by direct injection and the relative amount of fuelingby port fuel injection could increased as a function of time as theengine and catalyst warm up. Information on the fuel composition (mostlyalcohol content) can be used to adjust the amount of directly injectedfuel.

The increased role of port injection as a function of time would bedetermined by the tradeoff between the benefit of using direct injectionfor better fuel control through precise injection of the amount of thefuel that is needed for near stoichiometric combustion and the detrimentof the greater larger inhomogeneity of the air fuel mixture. Port fuelinjection results in improved mixture uniformity while at the same timeless precise control of the fuel injection, as the fuel transport occursthrough the film established near the inlet valve or valves. Therelative amounts of port fuel and direct fuel injection would beoptimized as a function of time after the fuel has been ignited so as tominimize particulate emissions. This optimization could be facilitatedby allowing a greater relative amount of port fuel injection, whenneeded, by increasing spark retard.

The use of multiple points and forms of fuel introduction into thecylinder is useful during these times. For example, during cold startoperation, the control system could call for direct injection for a fewcycles during the startup, for precise metering of the fuel, followed byincreased use of port fuel that could reduce the particulate emissions.Multiple injections from the direct injectors could be used to minimizethe production of particulates.

Engine Shutdown

Employment of an optimized combination of port fuel injection and directinjection during engine shutdown could also be employed. Operation forthe few cycles after engine shutdown without use of port injection couldreduce the film at the valves and reduces the amount of fuel in thepuddle near the inlet valves for the subsequent start.

Other Transients

The control approach described above could also be applied to certaintransient conditions during the rest of the drive cycle.

This strategy can also be used for transient operation in vehicles withdirectly injected spark ignition engines that use engine shutdown andrestart for improved efficiency; this stop-start operation is usedduring vehicle deceleration and idle to improve efficiency by not usingthe engine when it operates very inefficiently. The strategy can also beused for vehicles with full hybrid powertrains.

In addition to use at cold start and engine shutdown, the optimized portfuel injection-direct injection system discussed above can be useful foroperation with certain transients where there is rapid variation inengine speed or torque. This can be especially important at high enginespeeds. At high engine speeds, there are issues with mixture formationbecause of the shortened times, although there is increased turbulencein the cylinders. Direct injection should be minimized because mixtureformation issues resulting in particulate emissions. However, sinceoperation at high speed usually is associated with increasedtemperatures, this need could be reduced by increased particulate matteroxidation occur under these conditions.

Other Techniques

While direct injection may be useful for controlling hydrocarbon vaporemissions during cold start and other transients, these emissions can becontrolled with conventional means. In contrast, particulate emissions,if not sufficiently reduced by fuel management, could require use of agasoline particulate filter (GPF), which can result in significantadditional complexity and cost. Thus, it may be attractive for thecontrol systems to change the relative amounts of port and directinjected fuel so as to reduce particulate emissions at the expense ofmore hydrocarbon emissions.

Progress in fueling system technology can make it possible to controlthe fuel injected in each cylinder. Thus, instead of commanding all theinjectors to introduce the same amount of fuel, and same injector timingand other conditions, the fuel injection conditions for each cylindercould be controlled separately. It is known that there are variations ofperformance of different cylinders, and the computers can now controleach cylinder separately. It may be possible, for example, to run somecylinders at different stoichiometries than others, with an overallcorrect stoichiometry of the air/fuel mixture. It may also be possibleduring transients, such as cold start or hard acceleration, to changetemporarily the overall stoichiometry.

Another option is to use different fuels during cold startup andtransients, if available on board. Fuels that can be easily evaporated(fuels that have high vapor pressure, such as alcohols, low heat ofevaporation or fuels with low evaporation temperature, or fuels withmostly saturated carbon bonds) can be used to minimize particulateproduction. The fuel that can be easily evaporated or fuel that have lowpropensity for making soot (such as methanol or ethanol) can be usedduring these conditions to minimize particulate matter. Althoughmethanol and ethanol have relatively high heat of vaporization, theyhave relatively low propensity for making soot, and they have high vaporpressure. The fuels that can be easily evaporated, or the fuel that haslow propensity for making soot can be either provided through externalmeans to a separate container (i.e., external fill) or can be separatedonboard from a single fuel that is a mixture of gasoline and alcohol andstored in a separate container. Since only a small amount of fuel isrequired, the size of the tank and/or the capacity of the onboardseparating system can be relatively small.

The ratio of the amount of fuel that is port fuel injected to the fuelthat is directly injected can be varied according to the amount ofalcohol (ethanol or methanol), if any, that is mixed in with thegasoline. The use of alcohol can reduce particulate mass and number andthus reduce the relative amount of port fuel injection that is needed toreduce particulate emissions. The relative amount of port fuel injectioncan be decreased when alcohol is mixed with gasoline or the relativeamount of alcohol in a mixture with gasoline is increased.

The embodiments that have been described can also be utilized with asupercharged engine or with a direct injection engine that does not usepressure boosting. They can also be used with engines that are operatedwith a substantially stoichiometric fuel air ratio with or without EGR;and with engines operated with either rich or lean fuel air mixtures.

What is claimed is:
 1. A spark ignition engine having port fuelinjectors and direct injectors wherein, in cold start, the engine isfirst operated using direct injection alone; and wherein, during a partof cold start, both port injection and direct injection are used and theratio of fuel that is port fuel injected to fuel that is directlyinjected increases with the increasing time of engine operation.
 2. Thespark ignition engine of claim 1, wherein spark retard is used to allowan increase in the ratio of port injected fuel to directly injectedfuel.
 3. The spark ignition engine of claim 1, wherein an amount ofspark retard that is used in cold start is greater at the same torqueand speed than the amount of spark retard that is used in at least partof the engine operation after cold start.
 4. The spark ignition engineof claim 1, wherein the engine is first operated with a nearstoichiometric fuel air mixture using direct injection alone at thebeginning of cold start.
 5. The spark ignition engine of claim 1,wherein an amount of fueling that is provided by port fuel injection ascompared to an amount of fueling that is provided by direct injectionincreases as the engine temperature increases.
 6. The spark ignitionengine of claim 1, wherein the spark ignition engine uses agasoline-ethanol mixture.
 7. The spark ignition engine of claim 1,wherein the engine uses gasoline.
 8. The spark ignition engine of claim1, wherein variable valve timing is employed to allow an increase in theratio of port injected fuel to directly injected fuel.
 9. The sparkignition engine of claim 1, wherein a change in spark retard is used toallow a higher ratio of port injected fuel to directly injected fuel athigh engine speeds.
 10. The spark ignition engine of claim 1, whereinparticulate emissions from the engine are reduced at the expense ofincreased hydrocarbon emissions from the engine.
 11. A spark ignitionengine having port fuel injectors and direct injectors; wherein coldstart begins with direct injection alone; wherein port fuel injection isused during cold start; wherein increased spark retard is used in partsof a drive cycle to allow a higher ratio of port injected fuel todirectly injected fuel; and wherein increased spark retard is used toallow a higher ratio of port injected fuel to directly injected fuel athigh speeds and/or when there are transients in torque.
 12. The sparkignition engine of claim 11, wherein particulate emissions from theengine are reduced at the expense of increased hydrocarbon emissions incold start.
 13. The spark ignition engine of claim 11, wherein sparkretard is employed to increase the ratio of fuel that is provided byport fuel injection to fuel that is provided by direct injection duringat least part of cold start.
 14. The spark ignition engine of claim 11,wherein particulate emissions are increased by the lower temperaturesduring cold start relative to particulate emissions after cold start.15. The spark ignition engine of claim 11, wherein increased sparkretard is used to allow an increase in the ratio of port injected fuelto directly injected fuel during transients in torque.
 16. The sparkignition engine of claim 11, wherein increased spark retard adjustmentis used to allow an increase in the ratio of port injected fuel todirectly injected fuel at high speeds.
 17. The spark ignition engine ofclaim 11, wherein variable valve timing is also used to enable a higherratio of fuel that is port injected to fuel that is direct injected. 18.The spark ignition engine of claim 11, wherein direct injection alone isused during engine restart.
 19. The spark ignition engine of claim 11,wherein the use of increased spark retard to enable a higher ratio ofport injected fuel to directly injected fuel is used in engine restart.20. A spark ignition engine having port fuel injectors and directinjectors wherein, in cold start, the engine is first operated usingdirect injection alone; wherein port fuel injection is using during apart of cold start; wherein the engine is fueled with direct injectionalone at the beginning of engine restart; and wherein a change in sparkretard is used at selected times during a drive cycle to allow anincrease the ratio of fuel that is introduced by port fuel injection tofuel that is introduced by direct injection.
 21. The spark ignitionengine of claim 20, wherein during a part of cold start, both port fuelinjection and direct injection are employed and a greater ratio of fuelprovided by port fuel injection to fuel provided by direct injection isused at a given values of torque and speed than the ratio that is usedduring operation of the engine after cold start.
 22. The spark ignitionengine of claim 20, wherein a greater amount of spark retard is usedduring cold start than in engine operation after cold start.
 23. Thespark ignition engine of claim 20, wherein direct injection alone isused in engine shutdown.
 24. The spark ignition engine of claim 20,wherein during engine restart, fueling by direct injection alone isfollowed by the introduction of fuel by port fuel injection.
 25. Thespark ignition engine of claim 20, wherein spark retard is increased soas to allow a higher ratio of fueling from port fuel injection tofueling from direct injection at high engine speeds.
 26. The sparkignition engine of claim 20, wherein spark retard is increased so as toallow a higher ratio of fueling from port fuel injection to fueling fromdirect injection during transients in torque.